Track async causal stack traces

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 7f85580c24ae48780918ff7f8048dbcd8bc39296..9bac3a1a6d30a62aee4ee59a2a4cb9c41be29368 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -22638,8 +22638,16 @@ const char* StackTrace::ToCStringInternal(intptr_t* frame_index,
   intptr_t total_len = 0;
   GrowableArray<char*> frame_strings;
   for (intptr_t i = 0; (i < Length()) && (*frame_index < max_frames); i++) {
+    code = CodeAtFrame(i);
     function = FunctionAtFrame(i);
-    if (function.IsNull()) {
+    if (code.raw() == StubCode::AsynchronousGapMarker_entry()->code()) {
+      const char* kAsynchronousGap = "<asynchronous suspension>\n";

[rmacnak, 2017/01/26 18:05:58]

No need to dup the string.

[Cutch, 2017/01/31 23:45:31]

Done.

+      intptr_t asynchronous_gap_len = strlen(kAsynchronousGap) + 1;
+      char* chars = zone->Alloc<char>(asynchronous_gap_len);
+      OS::SNPrint(chars, asynchronous_gap_len, "%s", kAsynchronousGap);
+      frame_strings.Add(chars);
+      total_len += asynchronous_gap_len;
+    } else if (function.IsNull()) {
       // Check for a null function, which indicates a gap in a StackOverflow or
       // OutOfMemory trace.
       if ((i < (Length() - 1)) &&
@@ -22655,7 +22663,6 @@ const char* StackTrace::ToCStringInternal(intptr_t* frame_index,
         (*frame_index) += Smi::Value(PcOffsetAtFrame(i));
       }
     } else {
-      code = CodeAtFrame(i);
       ASSERT(function.raw() == code.function());
       uword pc = code.PayloadStart() + Smi::Value(PcOffsetAtFrame(i));
       if (code.is_optimized() && expand_inlined() &&